UFC 3-420-01
25 October 2004
Air Force Projects, a minimum of three pumps will be provided. The third
pump will be a standby unit. Each pump is sized to meet the requirements of
the facility. Pump capacities in gallons per minute (L/s) will be in accordance
with Table 606.5.11-1. Pump head is to be equal to the high pressure
maintained within the hydro-pneumatic tank.
606.5.11.3 Tank Capacity. Tank capacity is to be based upon a withdrawal,
in gallons (liters), of 2-1/2 times the gallon per minute (L/s) capacity of the
pump and a low water level of not less than 10 percent of total tank capacity
or 3 inches (76 mm) above top of the tank outlet, whichever is greater. Table
606.5.11-2 indicates high water levels and withdrawals for efficient operation
of tanks with bottom outlets and a 10-percent residual. Using this table, the
tank capacity may be determined as per Example 1. Pressure ranges are
given in pounds per square inch (psi) and kilopascals (kPa)
Example 1. Determine the tank capacity when pump capacity is 150
gallons per minute and tank operating pressure range is 40 to 60 pounds
per square inch. (Referring to Table 606.5.11-2, the withdrawal from the
tank is 24 percent of the tank capacity.)
Total tank capacity = 2.50 x 150 gpm / 0.24 percent = 1,563 gallons
or
Total tank capacity = 2.50 x 568 L/s / 0.24 percent = 5916 liters
606.5.11.4 Compressed Air. Compressed air is supplied for tank operation
according to the tank capacities. Satisfactory operation has been attained by
providing 1.5 cubic feet per minute (cfm) for tank capacities up to 500 gallons
(1893 L) and 2 cfm for capacities from 500 to 3,000 gallons (1893 to 11355
L). For each additional 3,000 gallons (11355 L) or fraction thereof, add 2 cfm
(0.0566 cubic meters/min.). (Quantities are expressed in cubic feet (cubic
meter) per minute free air at pressure equal to the high pressure maintained
within the hydro-pneumatic tank.)
606.5.11.5 Controls. The controls of a hydro-pneumatic system are to
maintain the predetermined pressures, water levels, and air-water ratio within
the tank. When duplex pumps are provided, controls are to start only one
pump at a time. Pumps are to be operated alternately and run simultaneously
only when a single pump cannot maintain the predetermined low pressure. A
manual transfer switch will operate the third pump if provided. Controls are to
admit compressed air into the tank only when tank pressure at high water
level is below normal.
606.5.11.6 Booster Pumps. Booster pumps may be the "on-off" type or
continuous running type.
606.5.11.6.1 On-Off Type. The installation of an "on-off" type of pumping
system should be considered when relatively long periods of pump-on or
pump-off are anticipated. Pumps are to be activated, only when pressure
is inadequate, by a sensing device located in the pump suction line. Flow
normally is through a single full-size pump bypass with check valve and
A-17
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